I'm in the process of measuring and assembling my m20 2.7i build. All initial measurements were done with the engine hanging on an engine stand.
I've been pretty happy with the bearing measurements, but I noticed the #4 cylinder/piston was tighter than the rest - I could feel some drag that really worried me. When the block was machined, the shop only opened 1 of the 6 piston boxes. So I was worried that manufacturing tolerances across pistons may have resulted in the cylinder bore being too tight for some of them. So I took the block and pistons back to the shop to measure with their bore gauge and mic set.
With the block sitting on a cart, we measured 0.0013 - 0.0015 inch piston to cylinder clearance. Incidentally, this was the same piston that was used for initial measurements when the block was bored/honed. The Bentley gives a spec of 0.0004 - 0.0016 for a new piston and 0.0047 for the wear limit. This puts my pistons at the top end of the target spec, good for the added heat of boost.
The machinist says it makes sense that I was getting drag on the piston if I was assembling on a stand because the whole block would be distorting. They only build on a flat table.
So now my question, how much of the assembly needs to be on a flat?!? How much am I overthinking this?
I've been pretty happy with the bearing measurements, but I noticed the #4 cylinder/piston was tighter than the rest - I could feel some drag that really worried me. When the block was machined, the shop only opened 1 of the 6 piston boxes. So I was worried that manufacturing tolerances across pistons may have resulted in the cylinder bore being too tight for some of them. So I took the block and pistons back to the shop to measure with their bore gauge and mic set.
With the block sitting on a cart, we measured 0.0013 - 0.0015 inch piston to cylinder clearance. Incidentally, this was the same piston that was used for initial measurements when the block was bored/honed. The Bentley gives a spec of 0.0004 - 0.0016 for a new piston and 0.0047 for the wear limit. This puts my pistons at the top end of the target spec, good for the added heat of boost.
The machinist says it makes sense that I was getting drag on the piston if I was assembling on a stand because the whole block would be distorting. They only build on a flat table.
So now my question, how much of the assembly needs to be on a flat?!? How much am I overthinking this?
- I can easily get the crank installed while the block is on upside down on my precision granite flat.
- Pistons get fucking tricky because they sit proud of the block. Its going to be really tough to flip the block as it gets heavier. Plus, I can't see this being critical since the pistons will just flop into place.
- I can see it being important to get the head on while the block is flat to prevent distortion, but it's going to be tough to get the block to sit flat with the crank protruding.
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